Method of cooling glass



NOV. 26, 1935. Q MCCAULEY 2,022,215

METHOD OF COOLING GLASS Filedy Feb. 28, 1935 2 sheets-sheet 1 l I l L lINVENTOR; 6201765 K/VSZAWV I ATTORNEYS.

N0v.26, 1935. @v MCcAULEY f 2,022,215

METHOD QF COOLING `GLASS y I Filed Feb. 28, 1935 2 sheets-sheet 2INVENTOR. 620/565 /f/Vfza 7 ATTORNEYS.

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Patented Nov.' 26, 1935 UNITED STATES METHOD 0F COOLING GLASS vGeorge V.McCauley, Corning, N. Y., assignor to Corning Glass Works, Corning, N.Y., a corporation of New York Application Febmaryzs, 1935, serial No.8,755

9 Claims.

This invention relates to a method of cooling large masses of moltenglass and has for its principal object to prevent devitrication orcrystallization thereof during the cooling.

Another object is to prevent the formation of vacuum bubbles or voidsWithin the body of a large mass of molten glass during cooling.

Most glasses which are high in silica will devitrify or crystallize tosome extent at least with separation of crystals of` silica if the glassbe held for a sufliciently long time within a range of temperatureswhich will be designated herein as the crystallization range. Suchcrystallization occurs primarily at or near the surface of the glass andthe heterogeneous structure resulting therefrom is particularlyobjectionable in glass castings which are to be used for telescopemirrors because, when polished, the difference in hardness and expansioncoefficient of the silica crystals and the surrounding glassproduces asurface which is uneven and which is sensitive to the distortion due totemperature changes.

Rapid cooling ,through the crystallization range will generally preventthe occurrence of serious devitrication in this range of temperaturesand such treatment may successfully be applied to articles which arethin enough to cool at a substantially uniform rate throughout. However,vacuum bubbles sometimes occur in the case of large molten masses whichare so thick that, when subjected to rapid cooling, their interiorportions do not reach the temperature of solidicatlon until long afterthe exterior portions have become set and rigid. Therefore, in themanufacture of large glass castings such as a 200 inch telescope mirror,the rapid dissipation of the vast amount of heat which is contained inthe molten mass of glass constitutes a serious problem. I have foundthat the ideal way of cooling such large masses of molten glass down tothe temperature atwhich annealing may be begun would be to cool theinterior faster than the exterior. or, in other words, to cool in suchmanner that the exterior or upper portion of the casting is maintainedat a higher temperature than the interior or lower portion which isnea-rer to the bottom of the mold. Since the refractory mold itselfconstitutes an insulating medium for the lower part of the casting whichprei vents it from cooling as rapidly as the upper and exposed portion,the above stated ideal condition appears to be impossible to attain.

In spite of the technical diiliculties involved,v

I have devised a method whereby I am able to cool thick masses of moltenglass in a manner which prevents both ldevitrlfication andl theformation of vacuum bubbles and at the same time dissipates the excessheat rapidly and conveniently.

I have found that I can approach the above i mentioned ideal conditionin a practicable manner by ladling the molten glass into a relativelycool mold of refractory material so that the lower portion of thecasting will impart some of its heat to the mold and will be cooler thanthe upl0 per portion when the casting is completed. The casting is thenpermitted to cool rapidly until the surface thereof is congealed and isat a tempera, ture below the softening point but above the annealingpoint at which temperature the surface 1l is held, if need be, byapplying sucient heat. When the lower portion of the casting has cooledpractically to the same temperature the exposed surface is rapidlyreheated to a temperature above the softening point of the glass,thereby re- I) lieving internal stresses and the casting is furthercooled and annealed in the usual manner.

The temperature at which crystallization of the glass occurs mostrapidly will depend largely upon the composition thereof and correspondsto I6 the so called liquidus. I have found that for low expansionglasses of high silica content the crystallization range isapproximately 1050* C. to 1100 C., while for ordinary glasses of thesoda lime or lead type, this range is approximately 935 C. to 950 C.Cooling should proceed as rapidly as possible through thecrystallization range and down to the softening point of the glass,which latter is defined as that vtemperature at which a rod of the glass1 mm. in 85 diameter and 23 cm. long will elongate by its own weight atthe rate of 1 mm. per minute when heated throughout the upper 9 cm. ofits length.

As an example ofthe manner in which my invention is to be carried out, Igive the following 40 procedure of casting and cooling a disc having adiameter of 200 inches and a thickness of about 26 inches, which iscomposed of a high silica glass having an expansion coefficient of.05245, a softening point of about 830 C. and an annealing v point ofabout 520 C., the ,same being illustrated by the accompanyingdrawlngs'in which:

Fig. 1 is an elevation of an apparatus for casting and cooling largetelescope discs consisting of a` heating kiln, a refractory mold and anannealing kiln;

Fig. 2 is an elevation partly in section of the heating kiln andrefractory mold shown in Fig.

Fig. 3 is an elevation partly in section of the annealing kiln shown inFig. 1.

In the drawings, in which corresponding parts are indicated bycorresponding marks of reference, a heating kiln designated generallyJIII, provided with gas burners II and door-opening I2 for introducingladles (not shown), is suspended by supports I3 directly over arefractory mold designated generally I4. The bottom of the mold I4 isprovided with a plurality of built-in electrical heating elements I5 andupwardly-projecting refractory cores I6. The mold is supported upon afour-legged screw hoist I1, which in turn is mounted on` a four-wheeledcar I8 adapted to move along a track I9. An upper thermocouple 20projects through the top of the mold wall and a lower thermocouple 2I isinserted through the mold wall at a point lower down so that its hotjunction is adjacent the inner surface of the wall near the bottom ofthe mold.

At a distance of several feet from the kiln Il and substantially on alevel therewith an annealing kiln designated generally 22 is suspendedby supports 23 above a continuationof the track I9. The interior of thekiln 22 is provided in its top with a plurality of electrical heating`elements 24 and around the wall with heating elements 25. Suspended forvertical movement -within and near the top of the kilny 22 is a largerefractory mold cover 26 which is slightly smaller in size than theinside of the kiln. The bottom of the kiln 22 is open and is adapted toreceive the mold I4, which, on being inserted therein as shown in Fig.3, will contact and support upon its walls the refractory cover 2S, thelatter then serving to protect the heating elements 24 from the radiantheat Aof the molten glass in the mold I4. In this position it will beseen that the mold is entirely surrounded by electrical heatingelements.

The refractory mold I4 is preheated in the kiln I0 by means of theburners I I until the temperature at the upper thermocouple 20 is about800 C. at which time the temperature of the lower thermocouple 2| isabout 650 C. From the tank or melting container heated to about 1550 C.the glass is then ladled into the mold at the rate of about one ladlefulor 3 cu. ft. per 4 minutes until the mold is full. The first incrementsof glass are rapidly chilled by contact with the relatively cool moldand impart some of their heat thereto. The exchange of heat between theglass and the mold continues as more glass is added, the succeedinglayers of glass remaining relatively hotter than the lower ones onaccount of the low thermal conductivity of glass. The temperature of thekiln above the mold also rises as the casting proceeds and, when. thetemperature, measured by the upper thermocouple 2l, reachesapproximately 1100 C., the re in the kiln I0 is reduced-by regulatingthe gas burners I I so as to maintain this condition until casting iscompleted. When the mold I4 has thus been filled with the molten glass,the temperature registered by the lower thermocouple 2l is about 900 C.In order to permit the molten glass to settle and fine and to heat thesurface thereof above the crystallization point, the re in the kiln isthen increased until such time as the upper thermocouple 20 registersabout 1250 C. 0r higher. At`

this time the reading of the lower thermocouple 2l is about `1000 C. There of the kiln is then completely shut off and the kiln doors -openedWide to permit the casting to cool freely and as rapidly as possible.When the kiln above the mold reaches such a temperature that the upperthermocouple 20 registers about 575 C., the kiln doors are closed andthe fire is again applied thereto to increase the kiln temperature toabout 5 750 C., or to just below the softening point, as indicated bythe upper thermocouple 20. Such rapid cooling causes the upper layer ofthe casting to congeal and also causes the inner and lower portonthereof to cool more rapidly than 1o would otherwise be possible. Thetemperature which is indicated by the upper thermocouple 20 isthereafter maintained constant at about 750 C. until the lower portionof the casting, as indicated by the lower thermocouple 2| has cooled 15approximately to the same temperature and has therefore practically'entirely congealed. .-The shrinkage of the interior of the casting withrespect to the exterior, which occurs as solidifcation proceeds, sets upa condition of tensional 20 stress in the interior and lower portions ofthe casting which, if maintained'as was customary inprior methods, wouldtend to form vacuum bubbles and also particularly voids in thoseportions adjacent the cores I6. In order to permit the 25 exterior toyield and to relieve the tensional stress, the fires are again increasedand the temperature of the upper portion of the casting, as indicated bythe upper thermocouple I 6 is raised as rapidly as possible to about 900C., or Well 30' above the softening point.l During this flash heatingthe temperature of the interior portion of the casting is not raisedappreciably on account of the low thermal conductivity of the glass andthe casting is therefore in the most advan- 35E tageous condition asregards interior or exterior temperature difference for beginning theannealing treatment.- The mold I4 with its casting is then removed fromthe kiln I0 and transferred to the annealing ki1n'22, the bottom of themold 40- with its heating elements I5 constituting a closure for thebottom of the kiln 22. By means of the heating elements 24, 25, and I5which are controlled by automatic electrical switches and rheostats (notshown), the casting is further 45T cooled and annealed in the mannerknown in th art.

Since the difference vbetween the above mentioned crystallization rangesof low expansion,A high silica glasses and the average Soda lime or 50`lead glasses is about 100 degrees and the difference between theirrespective softening points is of approximately the same magnitude, itfollows that when the above described method of cooling is applied tolarge castings of lime or lead 55'v glass, the temperature pointsreferred to in the description will in each instance be approximately100 degrees lower than those employed for cooling the low expansionglass. However, on account of the higher expansion of the lime or leadglass, 60 such glasses cannot kbe so severely chilled as to bring themtoa corresponding distance below their softening points because thegreater amount of shrinkage which is inherent in these glasses of highexpansion tends to cause cracking. 65rv Therefore, in the case of suchglasses, the lowest temperature to which they are brought prior to thereheating step should be only slightly below their respective softeningpoints.

y I claim: 7o

1. The method of cooling a large mass of molten glass, which includesrapidly cooling it through its crystallization range to a temperaturebelow its softening point but above its annealing point, reheating. theexposed surfacel of 4aoaasaus the mass to permit the exterior to yieldand compensate the shrinkage of the interior and annealing the mass.

2. The method of cooling a large mass of molten glass, which includesrapidly cooling it through its crystallization range to a temperaturebelow its softening point but above its annealing point, reheating theexposed surface of the mass to a temperature about degrees above itssoftening point and further cooling and annealing the mass.

3. The method of cooling a large mass of molten glass, which includesrapidly cooling it through its crystallization range until the exposedsurface is at a temperature below its softening point but above itsannealing point, holding the external temperature the same until themass is substantially at the same temperature throughout, reheating theexposed surface to permit the exterior to yield and compensate theshrinkage of the interior and annealing the mass.

4. The method of cooling a large mass of molten glass, which includesrapidly cooling it through its crystallization range until the exposedsurface is at a temperature below its softening point but above itsannealing point,l holding the external temperature the same until themass is substantially at the same temperature throughout, reheating theexposed surface to a temperature about 100 degrees above its softeningpoint, and further cooling and annealing the mass.

5. The methodof cooling a. large molten mass of low expansionborosilicate glass, which includes rapidly cooling it through itscrystallization range until. the exposed surface is at a temperatureabout 200 degrees below the softening point of the glass, reheating theexposed surface until its temperature is only about v50 degrees belowthe softening point, holding the external temperature at the lattertemperature until the mass is substantially at the same temperaturethroughout, further reheating the exposed surface to a temperature about100 degrees above the softening point and cooling and annealing. themass. l

6. The method of making a large 4glass casting, which includesintroducing molten glass at a predetermined rate into a relatively coolmold so that the lowermost and interior portions of the casting arecooler than the upper portion, the exposed surface of the finishedcasting being above the crystallization temperature t the glass, coolingthe mass rapidly through the crystallization range to a temperaturebelow the softening point but above the annealing point, reheating theexposed surface to permit the exterior to yieldand compensate theshrinkage 5 e of the interior and annealing the mass.

'7. The method of making a large glass casting, which includesintroducing molten glass at a predetermined rate into a relatively coolmold so that the lowermost and interior por- 10 tions of the casting arecooler than the upper portion, the exposed surface of the finishedcasting being above the crystallization temperature of the glass,cooling the mass rapidly through the crystallization range until theexposed surface 15 is at a temperature below its softening point butabove its annealing point, holding the external temperature the sameuntil the mass is substantially at the same temperature throughout,reheating the exposed surface to permit the ex- 20 terior to yield andcompensate the shrinkage of the interior and annealing the mass.

8. The method of making a large glass casting, which includessuccessively introducing predetermined amounts of molten glass into arel- 25 atively cool mold at such a rate that the lowermost and interiorportions of the casting are cooler than the upper portion, the exposedsurface of the finished casting being above the crystallizationtemperature of the glass, cooling the 30 mass rapidly through thecrystallization range to a temperature below the softening point butabove the annealing point, reheating the exposed surface to permit theexterior to yield and compensate the shrinkage of the interior, and an-35 nealing the mass.

9. The method of making a large glass casting, which includessuccessively introducing predetermined amounts of molten glass into arelatively cool mold at such a rate that the lower- 40 most and interiorportions of the casting are cooler than the upper portion, the exposedsurface of the finished casting being above the crystallizationtemperature of the glass, cooling the mass rapidly through thecrystallization range until the exposed surface is at a temperaturebelow its softening point but above its annealing point, holding theexternal temperature the same until the mass is substantially at thesame temperature throughout, reheating the exposed sur- 50 face topermit the exterior to yield and compensate the shrinkage of theinterior, and annealing the mass.

GEORGE V. MCCAULEY.

